1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, an optical pickup apparatus for performing an information reading operation, and the tracking and focusing control operations.
2. Description of the Prior Arts
Data recording/reproducing apparatuses using optical disks such as a laser disk (LD) and a compact disk (CD) have been commercially available in recent years. There has been developed the optical pickup apparatus in which a laser beam is irradiated onto a data recording track (hereinafter referred to as a "track"), and data are reproduced based on the beam reflected by the track in order to read out data from an optical disk.
When the tracks are helically formed on the optical disk, since the sectors of a single track are not equidistant from the center of the rotation of the disk due to the eccentricity in the disc, tracking (radial) control is necessary in the read mode accurately irradiate the track with a laser beam. Even if the tracks are concentrically formed, sectors of a single track are not equidistant from the center of the rotation of the disk due to the eccentricity in the disk, and therefore, tracking control is essential. This tracking control has been conventionally performed by one beam method (a push-pull method) using single laser beam to read out the information (U.S. Pat. No. 4,767,921 issued to Ryoichi) or a three beams method (out-trigger method) wherein the single laser beam is separated into three beams by the diffraction grating and so on to read out the information (U.S. Pat. No. 4,973,886 issued to Matsuoka and U.S. Pat. No. 5,073,888) issued to Takahashi, etc).
That optical pickup apparatus is constructed in such a manner that one beam or three beams which have been formed from a beam irradiated from the laser source by the one beam method or three beams method, may be reflected by or transmitted through the optical disk to derive the tracking error signal, and so the tracking actuator may be operated in response to this tracking error signal to perform cooperative tracking.
Meanwhile, since the distance between the optical pickup and the disk shifts minutely in the read mode wherein the disk is being rotated, it is difficult to correctly read out data due to the shift, thus rendering focusing control essential. This focusing control has been conventionally performed by astigmatic method using an astigmatism phenomena (U.S. Pat. No. 4,684, 799 issued to Takanori) or a knife edge method (U.S. Pat. No. 4,684,799 issued to Masami et al., U.S. Pat. No. 4,868,377 issued to Kiochiro), etc.
The optical pickup apparatus is constructed in such a manner that a beam irradiated from the laser source may be reflected by or transmitted through the optical disk to derive the focusing error signal, and so the focusing actuator may be operated in response to this focusing error signal to perform cooperative focusing.
The conventional optical pickup apparatus will be described in detail with reference to FIG. 1.
FIG. 1 is a schematic view for showing a conventional optical pickup apparatus. A light source 10 is provided with one laser diode to emit a laser beam. In the case of the three beams method, a grating 18 is provided at the front or the rear of a collimator lens 11, that is, between laser source 10 and collimator lens 11 or between collimator lens 11 and a beam splitter 12, which separates one beam into three beams. The laser beam irradiated from laser source 10 is changed into parallel beams by collimator lens 11. These parallel beams, is the case of the three beams method, are separated into three beams by grating 18 and then are transmitted through beam splitter 12, a .lambda./4 plate 13, and an objective lens 14 to be incident upon the surface R of the disc D with a beam spot of about 1 .mu.m.
Beam splitter 12 has two right-angled prisms of which oblique (45.degree.) facets meet with each other. A polarizing film is formed on the oblique facet. Thus, with ensuring the straight property of the incident beam, a part of the incident beam is transmitted through the prisms and the other part is reflected from the polarizing film at an angle of 90.degree. together with the incident beam. Additionally, .lambda./4 plate 13 serves to avoid interference of the incident beam and the reflective beam, and converts a linearly polarized light beam into a circularly polarized light or vice versa.
The intensity of the light reflected from the disk D depends on the existence of a pit thereof having recorded data. The recorded information is read out on the basis of the intensity of the reflected light. The reflected light is transformed into the parallel light while going through objective lens 14, and polarized by 90.degree. by .lambda./4 plate 13, and then is incident onto beam splitter 12. In this beam splitter 12, one part of the incident beam is reflected to be bent through 90.degree.. A converging lens 15 is placed at the optical path of the reflected light to converge the reflected light. The reflected light converged by converging lens 15 goes through a cylindrical lens 16 (or knife edge) and then is received by a light receiving element (photodetector) 17 which is divided into four or six regions. The position errors of the pickup apparatus with respect to the disk D including a focusing error and a tracking error are detected from the state of the beam received on light receiving element 17, and focusing error signals and tracking error signals are generated on the basis of these focusing and tracking errors. A voice coil motor 19 as the objective lens actuator is actuated to move the objective lens in response to these focusing and tracking error signals, so that focusing and tracking can be controlled. The information on the disk is reproduced on the basis of the intensity of the reflected light which is determined by pit (P) on the disk (D).
According to the conventional optical pickup apparatus, so many optical constituent elements are required for reading information signals written in a recording medium and for controlling focusing and tracking, and therefore the structure of the optical pickup becomes complicated, the manufacturing cost becomes high, and the optical pickup apparatus becomes large. Further, it is difficult because so many optical constituent elements should be positioned precisely and minutely according to the optical path.
A hologram pickup apparatus as an example of another conventional optical pickup apparatuses has been suggested, which comprises a laser source for generating a laser beam, a diffraction grating for separating the laser beam into three beams for controlling tracking, a beam splitter for changing optical paths, and a hologram element which serves as a sensing lens and a light detecting element and forms an astigmatism for focusing. The hologram pickup apparatus is simpler in optical path than the optical pickup apparatus as shown in FIG. 1, and therefore its size and weight can be grately reduced, but still the hologram pickup apparatus has a troublesome problem in that optical constituent elements such as the hologram element and the laser source must be positioned precisely and minutely according to the optical path.